Crack closure

About: Crack closure is a research topic. Over the lifetime, 28157 publications have been published within this topic receiving 588158 citations.

The role of elastic anisotropy, length scale and crystallographic slip in fatigue crack nucleation

Abstract: Fatigue crack nucleation in polycrystal ferritic steel is investigated through experimental observation of multiple large-grained, notched, four-point bend tests combined with explicit microstructural representation of the same samples using crystal plasticity finite element techniques in order to assess fatigue indicator parameters, together with the roles of elastic anisotropy and length scale effects in slip development, and hence in crack nucleation. Elastic anisotropy has been demonstrated to play a pivotal role in the distribution and magnitude of polycrystal slip relative to observed crack nucleation sites in the context of constrained cyclic microplasticity. Length scale effects were found not to alter substantively the distributions or magnitudes of slip relative to the observed crack nucleation site, but in detailed analyses of an experimental sample, the location of highest magnitude of geometrically necessary dislocations was found to coincide precisely with the position of predicted peak plasticity and the experimentally observed crack nucleation site. The distributions of microplasticity within polycrystal samples were found to change quite significantly between the first yield and after multiple cycles. As a result, the effective plastic strain per cycle was found to be a better indicator of fatigue crack nucleation than peak effective plastic strain. In nine independently tested and analysed polycrystal samples, the cyclic effective plastic strain and crystallographic system peak accumulated slip were found to be good indicators of a fatigue crack nucleation site.

Environmental fatigue crack propagation of aluminum alloys at low stress intensity levels

Abstract: Environmental fatigue crack propagation in 2024-T3, 7075-T6, and 7178-T6 has been studied at low levels of cyclic amplitude of stress intensity, ΔK. Both wedge force loading and remote loading techniques were employed to achieve the desired ΔK levels, and preliminary experiments were designed to test their compatibility. Testing was carried out in humid air, distilled water, and 3.5 pct sodium chloride solution, and the observed crack growth rates compared with those in desiccated air. Later studies were also conducted in an inert reference environment with a total water content of less than 2 ppm. When the data are plotted as log ΔK vs log d2a /dN, alloy 2024-T3 exhibits a marked slope transition, alloy 7075-T6 a slight slope transition, and alloy 7178-T6 a rectilinear behavior throughout the whole range of ΔK studied. The basic shape of these curves is discussed in terms of state-of-stress conditions at the crack tip, frequency effects, environmental effects, strain rate sensitivity, and metallurgical structure. An attempt is also made to correlate the rate of fatigue crack propagation in a particular environment and at a particular ΔK level with the fracture topography.

Environmental crack and craze growth phenomena in polymers

Abstract: The nature of slow crack and craze propagation in polymers and crack propagation in inorganic glass has been considered in terms of time dependent processes By using a fracture mechanics analysis together with time dependent material parameters, equations have been derived to describe crack and craze propagation in both inert and active environments Experimental data from a range of materials suggest that a crack opening displacement (cod) criterion governs the crack propagation behaviour Incorporation of a simple fluid flow model into the cod analysis has allowed the failure processes in liquid environments to be described The data on organic polymers and inorganic glass suggest that when there are no problems of maintaining the environmental supply, the crack growth behaviour is controlled by relaxation processes in the material At high crack speeds a transition from relaxation to fluid flow control occurs when the time scale is too short for the liquid flow to be maintained The flow of an environment in long crazes can be shown to influence their behaviour in a similar manner, while under other conditions relaxation controlled craze growth can occur

Fatigue crack growth in metals and alloys: mechanisms and micromechanics

Abstract: Fatigue crack growth has been studied using several new experimental tools in the past ten years. The observation of fatigue cracks during growth under high resolution conditions has shown that crack advance is an intermittent process. These results, when combined with measurements of crack opening, displacements, crack closure, crack tip strains, fractography, and other information, leads to a reasonable understanding of many intrinsic aspects of fatigue crack growth at ambient temperature in a number of metallic alloys. Models of fatigue crack growth are reviewed from the perspective of this understanding. No model has achieved the capability of predicting fatigue crack growth from a description of microstructural and mechanical properties. The factors limiting a deeper understanding of fatigue crack growth are also more clearly defined, which gives some direction for future research. This paper is a critical review of crack growth mechanisms, mainly for large fatigue cracks subject to constant ...